Thermal method to control underfill flow in semiconductor devices

ABSTRACT

A method and apparatus for assembling a semiconductor device. A chip ( 901 ) with solder bodies ( 903 ) on its contact pads is flipped onto a substrate ( 904 ). After the reflow process, a gap ( 910 ) spaces chip and substrate apart. A polymer precursor is selected for its viscosity of known temperature dependence. The apparatus has a plate ( 800 ) with heating and cooling means to select and control a temperature profile from location to location across the plate. After preheating, the assembly is placed on a mesa ( 801 ) of the plate configured to heat only a portion of the substrate. Movable capillaries ( 840, 921 ) blow cooled gas onto selected locations of the assembly. After the temperature profile is reached, a quantity of the precursor is deposited at a chip side and pulled into the gap by capillary action. The capillary flow is controlled by controlling the precursor viscosity based on the temperature profile, resulting in a substantially linear front, until the gap is filled substantially without voids.

FIELD OF THE INVENTION

The present invention is related in general to the field of electronicsystems and semiconductor devices and more specifically to structure andmethod of void-free underfilling the gap of flip-chip electronicassemblies.

DESCRIPTION OF THE RELATED ART

It is well known in the so-called flip-chip technology how to mount anintegrated circuit chip to a substrate such as a printed circuit boardby solder bump interconnections. After the assembly process, theintegrated circuit chip is spaced apart from the printed circuitsubstrate by a gap. The solder bump interconnections extend across thegap and connect contact pads on the integrated circuit chip to terminalpads on the printed circuit substrate to attach the chip and thenconduct electrical signals, power and ground potential to and from thechip for processing. Similarly, after packaged semiconductor devices ofthe ball grid array families have been assembled to motherboards bymeans of solder bumps connections, the bumps define a gap between thepackaged device and the board.

There is a significant difference between the coefficient of thermalexpansion (CTE) between the semiconductor material used for the chip andthe material typically used for the substrate; for instance, withsilicon as the semiconductor material and plastic FR-4 as substratematerial, the difference in CTE is about an order of magnitude.

As a consequence of the CTE difference, mechanical stresses are createdwhen the assembly is subjected to thermal cycling during use or testing.These stresses tend to fatigue the solder bump interconnections,resulting in cracks and thus eventual failure of the assembly. In orderto strengthen the solder joints without affecting the electricalconnection, the gap is customarily filled with a polymeric material(containing inorganic fillers), which encapsulates the bumps and fillsany space in the gap between the semiconductor chip and the substrate.For example, in the well-known “C-4” process developed by theInternational Business Machines Corporation, viscous polymericprecursors are used to fill the space in the gap between the siliconchip and the ceramic substrate (see also IBM J. Res. Develop., vol. 13,pp. 226-296, 1969).

The polymeric precursor, sometimes referred to as “underfill”, istypically applied after the solder bumps are reflowed to bond thesemiconductor device to the substrate. The underfill is dispensed ontothe substrate adjacent to the chip and is pulled into the gap bycapillary forces. Thereafter, the precursor is heated, polymerized and“cured” to form an encapsulant. These approaches become increasinglyinsufficient as the number of bump interconnections increases and thebump size and the bump center-to-center pitch shrink. With these trends,the number of voids in the underfill and the risk of clustering thefillers in the precursor increase sharply.

SUMMARY OF THE INVENTION

Applicants recognize the need for an assembly methodology, materialpreparation and fabrication technique that provide not only stress-free,but also void-free underfilling. The methodology is based on the carefulcontrol of the parameters determining the phenomenon of capillarity.

One embodiment of the invention is a method for assembling asemiconductor device. A chip with reflow bodies on its contact pads isflipped onto a substrate. After the reflow process, a gap spaces chipand substrate apart. A polymer precursor is selected for its viscosityof known temperature dependence. An apparatus is then provided, whichhas a plate with heating and cooling means to select and control atemperature profile in time and from location to location across theplate. After preheating, substrate and chip are placed on the plate forreaching the temperature profile. A quantity of the precursor isdeposited at a chip side and pulled into the gap by capillary action.The capillary flow is controlled by controlling the viscosity based onthe temperature profile. As a result, the polymer progresses in asubstantially linear front, until the gap is filled with polymersubstantially without voids.

Another embodiment of the invention is an apparatus with a plateoperable to acquire a surface temperature of controlled profile. Insidethe plate are means for local heating, and across the plate surface aremovable capillaries, which have nozzles of controlled openings suitablefor air flow. Air of controlled temperature can be pressured through thecapillary nozzles. Temperature sensors are distributed in the plate,operable in a feedback mode to select and control the temperature of theplate surface from location to location.

It is a technical advantage of the invention that the assembly processis simple and low-cost, applicable to large-chip semiconductor products,high numbers and small size of bumps, and fine bump pitch. At the sametime, the method is flexible and can be applied to a wide spectrum ofmaterial and process variations, leading to improved semiconductordevice reliability.

The technical advances represented by the invention, as well as theaspects thereof, will become apparent from the following description ofthe preferred embodiments of the invention, when considered inconjunction with the accompanying drawings and the novel features setforth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic X-ray top view of a semiconductor chip attached bysolder balls to a substrate, while underfill material moves through thechip-substrate gap in an irregular front in known technology.

FIG. 2 is a schematic X-ray top view of a semiconductor chip attached bysolder balls to a substrate, while underfill material has filled thechip-substrate gap incompletely (causing a void) in known technology.

FIG. 3 is a schematic cross section of a chip attached by solder ballsto a substrate, illustrating the capillary action of filling the gapwith a viscous material.

FIG. 4 is a plot of the dynamic viscosity η of polymer precursors(logarithmic scale) as a function of temperature (linear scale).

FIG. 5 is a schematic X-ray top view of a semiconductor chip attached bysolder balls to a substrate, with an embodiment of high and lowtemperature zones causing underfill material to move through thechip-substrate gap in a regular front.

FIG. 6 is a schematic X-ray top view of a semiconductor chip attached bysolder balls to a substrate, with another embodiment of high and lowtemperature zones causing underfill material to move through thechip-substrate gap in a regular front.

FIG. 7 is a schematic plot of temperature versus location illustratingthe temperature profile across a semiconductor/substrate assembly.

FIG. 8 illustrates schematically a top view of a heatable plate withmesas of various shapes and inlets of cooling gas; the positions ofchips, substrates, and syringes are indicted by dashed/dotted lines.

FIG. 9 depicts schematically a cross section of the heatable plate withmesas, inlets for cooling gas, capillaries for cooling gas, and asyringe for precursors; the cross section is along line A1-A2 of FIG. 8.

FIG. 10 depicts schematically a cross section of the heatable plate withmesas, capillaries for cooling gas, and a syringe for precursors; thecross section is along line B1-B2 of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The schematic top X-ray views of FIGS. 1 and 2 illustrate a typicalexample of an underfill process in known technology of a semiconductorchip assembled on a substrate. Looking through the flip-assembledtransparent chip 101, FIGS. 1 and 2 show rows of solder connections 102connecting the chip and to the substrate 103 (also transparent). Thesolder connections 102 between chip 101 and substrate 103 create a gap.A quantity of liquid, viscous polymeric material 104 is deposited alongchip side 101a for filling this gap; capillarity pulls the polymer intothe gap and provides the mechanism for filling.

Capillarity is the capillary action by which the surface of a liquid,where it contacts a solid, is elevated or depressed, because of therelative attraction of the molecules of the liquid for each other andfor those of the solid. Capillary attraction is the force of adhesionexisting between a solid and a liquid in capillarity.

Capillary action becomes effective, when a positive force of adhesionexists between the wettable solid surface of a capillary tube or gap,and a wetting liquid inside the tube or gap (“capillary attraction”). Inan approximately horizontal tube or gap, the pulling force moves theliquid along the tube or gap.

FIG. 1 illustrates the problematic behavior of the liquid polymericmaterial 104 (dark color in the gap) in known technology. While aportion 104 a is being pulled into the chip-to-substrate gap, otherportions 104 b and 104 c flow along adjacent sides of the chip and startfrom different angles to be pulled into the gap. The arrows in FIG. 1indicate schematically this complex situation. As a consequence, themerging of the fronts from different angles results in an irregulareffective progression front, with the center portion of the front movingslower and the portions along the chip sides faster.

FIG. 2 depicts the substantial risk of trapping air pockets along theprogression. The accelerated portions 204 have merged; finally theyenclose air pocket 205. The captured void 205 causes stressirregularities and thus a high risk of device failure. Thesedifficulties are resolved by the embodiments of the invention.

For the assembly process of the invention and the design of theapparatus, the invention applies certain laws of fluid dynamics anddeformable medium and extends them to the complex conditions ofsemiconductor product fabrication.

For a deformable medium flowing in a gap having different cross sectionsq in various parts, continuity requires that the amount of deformablemedium flowing per unit of time through each cross section is directlyproportional to q and to the velocity v in this cross section:

qv=const.

In a gap, a deformable medium flows fastest at the smallest crosssection.

The velocity v of the flowing medium of density ρ is correlated to itspressure p after BERNOULLI by

½ρv ² +p=const.

The pressure of a flowing medium is the smaller the greater its velocityis. Consequently, the pressure at the smaller cross sections is smallerthan at the larger cross sections.

When the parts of the gap with different cross sections are separated bydifferent lengths L of the gap, one also has to consider the drop ofpressure along the gap lengths; the drop, in turn, depends on thecharacteristics of the flow, laminar versus turbulent.

A deformable medium flowing in a portion of a gap of radius r and lengthL at a velocity v, averaged over the gap portion cross section,experiences a pressure drop Δp due to friction. For idealizedconditions, such as neglecting the inertia of the flowing medium, HAGENand POISEUILLE have found for laminar flow

Δp=8ηLv/r ².

(η=dynamic viscosity)

The pressure drop of the medium along the gap portion length is directlyproportional to the first power of the average velocity and inverseproportional to the second power of the portion radius.

In contrast, for turbulent flow the relationship is

Δp=ρλLv ² /r.

(ρ=density, λ=dimensionless number related to REYNOLD's criteria oftransition from laminar to turbulent flow).

The pressure drop of the medium along the gap portion length is directlyproportional to the second power of the average velocity and inverseproportional to the first power of the portion radius.

FIG. 3 illustrates the capillary conditions in the gap of width 2r(determined by the height 2r of the solder connections 302) between chip301 and substrate 303. With these designations, the flow-out t is givenby:

t=(3η/γ cos Θ)(L ²/2r)

(γ=surface tension)

(Θ=contact angle)

The flow-out time is directly proportional to the square of the flowdistance and inverse proportional to the capillary width (after J. WANG,Microelectr. Reliab. 42, 293-299, 2002). The equation is onlyapproximately correct and does not comprehend the time dependence ofmaterial parameters. FIG. 3 also shows the meniscus 310 of the fillet atthe side edge of chip 301; it is also characterized by contact angles Θ.Analogous considerations hold, when 301 in FIG. 3 is a packagedsemiconductor device.

The above relations for the laminar flow and the flow-out time highlightthe importance of the dynamic viscosity η. By increasing the temperatureand thus decreasing the viscosity, the pressure drop of the flowingprecursor can be decreased, the velocity increased, and the flow-outtime decreased. The detailed dependence of the viscosity η (poise,logarithmic scale) on temperature (° C., linear scale) for severalpolymer precursor materials is plotted in FIG. 4. The viscosity dropssharply (about three orders of magnitude) in the temperature range fromambient temperature to about 130 to 140° C. Polymer precursors with aviscosity dependence as plotted in FIG. 4 are commercially available,for example from Ablestik Corp., Rancho Dominguez, Calif., USA.

The polymer preferably includes inorganic filler particles (such asalumina or silica) selected to strengthen the polymerized polymermechanically and reduce its coefficient of thermal expansion. Thelargest diameter of the filler particles is preferably less thanapproximately 30% of the gap, amplifying the regulating effect of theviscosity change.

The schematic top X-ray views of FIGS. 5 and 6 show embodiments of theinvention, a thermal method to control the flow of a polymer precursorin the underfilling process of a flip-assembled semiconductor chip withmetallic contact pads. In FIG. 5, looking through the flip-assembledtransparent chip 501 outlined by its sides 501a, a dense array of solderbodies 502 can be seen, which are attached to each contact pad andconnect chip 501 with substrate 503 (also transparent). The chip 501 ispreferably a semiconductor component such as a high input/output siliconintegrated circuit. The solder bodies are preferably balls or bumps madeof tin-based compounds (such as tin/silver, tin/copper, tin/bismuth, orternary compounds) with a reflow temperature higher than the temperatureused for controlling the precursor viscosity. The substrate 503 may beceramic or plastic, with or without lamination, or glass. The surface ofthe substrate has metallic terminals in locations matching the chipcontact pads.

In the process of attaching chip 501, with the solder bodies attached toits contact pads, to the substrate 503, chip and substrate are alignedso that each chip solder body is aligned with and touches the matchingsubstrate terminal. Thermal energy is then applied to reflow the solderbodies. The attached chip 501 and substrate 503 are cooled to apredetermined temperature (which may still be significantly higher thanambient temperature); an assembly between chip and substrate is thuscreated with a gap spacing the chip and the substrate apart. An exampleof the gap is indicated in FIG. 3.

Next, an adherent polymer precursor is selected, preferably filled withinorganic fillers, on the basis of its temperature dependent viscosity.The dependence, generally depicted in FIG. 4, permits the selection ofthe high and the low temperatures for the assembly zones to create theprecursor viscosity differences best suited for a precursor flow withlinear front through the assembly gap.

An apparatus is now provided, which has a heatable plate with a surfacecontoured into castellated mesas. The apparatus further includes severalcapillaries with openings formed to create a jet of streaming gas, whichpreferably is cooled to a temperature below ambient temperature. Inaddition, the apparatus has at least one syringe with a nozzle designedto dispense the selected filler-loaded precursor. The apparatus and itsvariations is described in more detail below. The plate is preheated toreach a predetermined temperature.

The chip-on-substrate assembly, as described above, is placed—substratedown—on a plate mesa, so that the heated mesa can transfer heat to thatportion of the substrate, which is intended to acquire the temperatureof the plate. In the embodiment of FIG. 5, the elevated temperature zone510 stretches over a center portion of the chip and has outlines 510 aabout parallel to the chip sides 501 a.

Next, the capillaries, which are movable in three dimensions, are movedabove the chip surface of the assembly and positioned over the shipsurface approximately parallel to the chip sides 501 a. Dependent on thesize of the chip, two or three or more capillaries are required on eachside of the chip. Alternatively, one or more capillaries may be movedalong the assembly side to face the gap (or one or more solder bodies).Gas such as dry nitrogen or dry air is then pressured through thecapillaries to create a stream of gas flowing out of the capillaryopening, hitting the chip surface; the gas streams at a controlled rate.Preferably, the gas is cooled, more preferably in the temperature rangefrom about −10 to +20° C.; a controlled stream of cooled gas at acontrolled temperature is thus hitting the chip surface, locally coolingthe chip and, by conductance through the solder bodies, the substrate.In FIG. 5, the cool temperature zones 511 stretch along the two chipsides 501 a and are juxtaposed the heated zone 510, creating atemperature profile across the assembly. The transition zone between theheated and the cooled zones stretches preferably between 1 and 6 mm (seeFIG. 7).

As illustrated in FIG. 9, another embodiment of the cooling capillariesemploys curved capillaries, which are shaped to provide cooled gas jetsdirectly into the gap, thus greatly enhancing the cooling effect, whichthe gas exerts on the surfaces of the gap.

Next, at least one syringe, which is movable in three dimensions, ismoved to chip side 501 b of the assembly. The syringe has an openingsuitable for dispensing a polymer precursor of a controlled temperature.A quantity 520 (dashed outline in FIG. 5) of the selected liquid viscousprecursor is deposited along chip side 501 b for filling the gap. Whilethe precursor is pulled into the gap by capillary action, its viscosityis modified by the temperature profile. Consequently, the precursormoves faster in the heated zone and slower in the cooled zones, wherethe capillary force is stronger. The result illustrated in FIG. 5indicates that the front 521 of the penetrating precursor issubstantially linear, and it keeps its linear shape for the fullcapillary movement through the gap. Consequently, there is no longer arisk of leaving a void behind or forming an enclosed volume of air (asin FIG. 2).

Alternatively, multiple syringes are moved to the chip side 501 b; apreferred number of syringes is three, as shown in FIG. 5. The syringeshave openings suitable for dispensing a polymer precursor. In oneembodiment, the syringes dispense the same precursor material, but thematerial 530 from the center syringe is at a higher temperature than thematerials 531 from the outer syringes; consequently, the precursorviscosity is lower in the center and higher in the regions of greatercapillary force. In another embodiment, the syringes dispense precursormaterials of different viscosities, or altogether different precursormaterials. It is preferred that the material 530 from the center syringehas a lower viscosity than the materials 531 from the outer syringes.The viscosities are higher in regions of greater capillary force;consequently, in all these modifications, the front 521 of thepenetrating precursor materials is substantially linear, and the frontkeeps its linear shape for the full capillary movement through the gap.

The method further includes the steps polymerizing the precursor at anelevated temperature, and then to cool the assembly together with theprecursor to ambient temperature.

FIG. 6 illustrates another embodiment with a modified zone of elevatedtemperature/lower viscosity and the resulting substantially linear flowof a polymer precursor in the underfilling process of a flip-assembledsemiconductor chip with metallic contact pads. In FIG. 6, lookingthrough the flip-assembled transparent chip 601 outlined by its sides601 a, 601 b, and 601 c, a dense array of solder bodies 602 can be seen,which are attached to each contact pad and connect chip 601 withsubstrate 603 (such as a ceramic or a plastic laminated substrate, buttransparent in FIG. 6). The surface of the substrate has metallicterminals in locations matching the chip contact pads.

When chip 601 and substrate 603 are connected by reflowing solder bodies602, a gap is created, which spaces the chip and the substrate apart.

Next, an adherent polymer precursor is selected, preferably filled withinorganic fillers, on the basis of its temperature dependent viscosity.The dependence, generally depicted in FIG. 4, permits the selection ofthe high and the low temperatures for the assembly zones to create theprecursor viscosity differences best suited for a precursor flow withlinear front through the assembly gap.

The apparatus provided has a heatable plate with a surface contouredinto castellated mesas. The mesas are configured with a surface of atrapezoidal outline: Of the four sides, two opposing sides are parallelto each other, while the two other sides are inclined against eachother. After heating the plate with the mesas, tapered zones 610 ofelevated temperature can be formed; the width of the zone tapers fromthe wide side 611 to the narrow side 612. The apparatus further includesseveral capillaries with openings formed to create a jet of streaminggas, which preferably is cooled to a temperature below ambienttemperature. In addition, the apparatus has at least one syringe with anozzle designed to dispense the selected filler-loaded precursor.

First, the plate is preheated to reach a predetermined temperature.Next, a chip-on-substrate assembly is placed—substrate down—on one ofthe mesas so that it can reach the temperature profile of FIG. 6. Thecooler zones 613 on each side of the chip start narrow on chip side 601b and become gradually wider towards chip side 601 c.

Next, the capillaries, which are movable in three dimensions, are movedabove the chip surface of the assembly and positioned over the shipsurface approximately following the trapezoidal mesa sides. For manychip sizes, three or more capillaries are required on each side of thechip. Gas such as dry nitrogen or dry air is then pressured through thecapillaries to create a stream of gas flowing out of the capillaryopening, hitting the chip surface; the gas streams at a controlled rate.Preferably, the gas is cooled, more preferably in the temperature rangefrom about−10 to +20° C.; a controlled stream of cooled gas at acontrolled temperature is thus hitting the chip surface, locally coolingthe chip and, by conductance through the solder bodies, the substrate.

Next, a syringe movable in three dimensions is moved to chip side 601 bof the assembly. The syringe has an opening suitable for dispensing apolymer precursor of a controlled temperature. A quantity 620 (dashedoutline in FIG. 6) of the selected liquid viscous precursor is depositedalong chip side 601 b for filling the gap. While the precursor is pulledinto the gap by capillary action, its viscosity is modified by thetemperature profile. Consequently, the precursor moves faster in theheated zone and slower in the cooled zones. The result illustrated inFIG. 6 indicates that the front 621 of the penetrating precursor issubstantially linear, and it keeps its linear shape for the fullcapillary movement through the gap. Consequently, there is no longer arisk of leaving a void behind or forming an enclosed volume of air (asin FIG. 2).

As illustrated in FIG. 7, it is preferred for many precursor materialsto profile the elevated temperature zone of the chip-and-substrateassembly (510 in FIG. 5, 610 in FIG. 6) so that the center portion fromx₂ to x₃ has an approximately flat temperature value at T₂ (in the 95 to110° C. range), while the adjacent cooler side ranges (511 in FIG. 5,613 in FIG. 6) are flat at a temperature T₁ (in the 60 to 85° C. range).Between the high and the low temperature regimes (between x₁ and x₂ andbetween x₃ and x₄) are temperature gradients; preferably the gradientshould be more than 5° C. over a distance of about 1 to 6 mm.

FIGS. 8, 9 and 10 depict views of the apparatus used for creating andcontrolling the temperature profile of a flip-chip-and-substrateassembly to provide a linear front in the underfilling process. FIG. 8represents a top view of the apparatus, combining a number of equipmentvariations in a single drawing; FIG. 9 is a cross sectional view alongline A1-A2, also including a number of apparatus options in one drawing;and FIG. 10 is a cross sectional view along line B1-B2.

FIG. 8 illustrates a (controllably heatable) metal plate 800 with abody, from which rows of mesas are protruding. Each mesa top is shapedto support at least a portion of the substrate of a semiconductorflip-chip assembly. In the preferred plate configuration, the surfacearea of the mesas is smaller than the area of the substrate assemblyplaced on the mesas. It should be pointed out that in order to createreproducibly the desired temperature profile in a selected semiconductorassembly, all mesas preferably have the same size and surface contour;the desired profile is created by the combination of heating and cooling(see below). However, in order to illustrate several design options,FIG. 8 includes more than one mesa surface design.

In FIG. 8, the mesas 801, 802, etc. in the first row have a rectangularsurface, the mesas 811, 812, etc. of the second row have a trapezoidalsurface. Other mesas may have different surface geometries; they are notshown in FIG. 8. For the mesa size selected, FIG. 8 also shows asuitable chip size 820 and substrate size 830.

In order to monitor the temperature for controlling the temperatureprofile, a plurality of temperature sensors may be distributed in plate800 and in the mesas; the sensors are not shown in FIG. 8.

In addition, FIG. 8 depicts a number of plate inlets 840 for blowingcooled gas from below the plate 800 onto the assembly-being-processed.The inlets may be provided for any mesa design (not only for rectangularmesas), and in any number and position. Furthermore, the preferredlocation of the syringe dispensing the polymer precursor is outlined bythe dashed circle 850.

FIG. 9 is a cross section of a portion of the heatable plate 800 alongline A1-A2 in FIG. 8. The mesas 801 and 802 protrude from the surface800 a of the plate. Near the mesas are openings 840, which are suitablefor blowing gas from the bottom through the plate towards an assemblypositioned on the mesa.

FIG. 9 illustrates a semiconductor assembly positioned on each of themesas; in FIG. 9, the assemblies belong to the same semiconductor type;alternatively, they may be different. The assembly on mesa 801 isdesignated 930, the assembly on mesa 802 is designated 940. In FIG. 9,both assemblies include a semiconductor chip 901 outlined by sides 901 aand metallic pads 902. Solder bodies 903, such as tin alloys, areattached to pads 902. The assemblies further include a substrate 904with surface 904 a and metallic terminals 905 in locations matching thechip pads 902. The solder bodies 903 are also attached to pads 905 sothat a gap 910 is created, which spaces chip 901 and substrate 904apart.

The apparatus of FIG. 9 further includes several capillaries 840. Thecapillaries are movable, individually or as a group, in x-, y-, andz-direction. In FIG. 9, the capillaries are positioned over assembly930; after completing the underfill step, they will be mover overassembly 940. The capillaries have a diameter and an orifice suitablefor pressing gas through the capillary to let it stream onto a surfacepositioned at a distance from the opening. In the example of FIG. 9, thegas jet hits the passive surface of semiconductor chip 901flip-assembled on substrate 904. Preferably, the gas is flowing at acontrolled rate, and is cooled to a temperature below ambienttemperature, preferably between about −10 and +20° C. In the preferredembodiment, the gas is dry nitrogen. From the cooled surface location,the cooling effect spreads through the semiconductor thickness of thechip and the solder bodies 903 to the substrate 904.

The capillaries may have straight shape, as shown by examples 840 inFIG. 9, or they may be curved as indicated by the examples 921. In thelatter configuration, the capillary orifice points towards the assemblygap 910 and thus enables the cooled gas to hit directly some solderbodies 903 close to the perimeter of the assembly and certain surfacelocations of the substrate 904, thereby significantly enhancing thecooling effect.

In dashed outline, FIG. 9 indicates the preferred position of thecapillary 850 used to dispense, at one face of the assembly, the polymerprecursor material needed for the underfilling process of the assembly.The precursor is at a controlled temperature. In another embodiment, theapparatus may have multiple (e.g., three) dispensing capillariespositioned side by side along the assembly face; they even may havedifferent orifices for different rates of dispensing (see the largerdashed circle 530 in FIG. 5 compared to the smaller circles 531). In afirst embodiment, the multiple (e.g., three) syringes are used for thesame precursor; the precursor in the center syringe, however, has ahigher temperature than the materials in the adjacent syringes. In asecond implementation, the syringes are used for different types ofprecursors. Preferably, the center syringe has a precursor type with alower viscosity than the precursor type in the adjacent syringes.

FIG. 10 is a cross section of a portion of the heatable plate 800 alongline B1-B2 in FIG. 8. The mesas 801 and 811 protrude from the surface800 a of the plate. FIG. 10 illustrates a semiconductor assembly,composed of a semiconductor chip and a substrate, positioned on each ofthe mesas. The assembly 930 (same as in FIG. 9) is placed on mesa 801,the assembly 1000 on mesa 811. In FIG. 10, assemblies 930 and 1000 areof the same device type; in other embodiments, they may be of differentdevice types. Solder bodies 1003, such as tin alloys, are connectingchips and substrate so that a gap 1010 is created, which spaces chipsand substrates apart.

The apparatus of FIG. 10 further includes the capillary 850, which isused to dispense, at the face 930a of the assembly, the polymerprecursor material needed for the underfilling process of the assembly.The precursor is at a controlled temperature. In another embodiment, theapparatus may have three dispensing capillaries positioned side by sidealong the assembly face 930 a; they even may have different orifices fordifferent rates of dispensing (see the larger dashed circle 530 in FIG.5 compared to the smaller circles 531). In a first implementation, thethree syringes are used for the same precursor; the precursor in thecenter syringe, however, has a higher temperature than the materials inthe adjacent syringes. In a second implementation, the syringes are usedfor different types of precursors. Preferably, the center syringe has aprecursor type with a lower viscosity than the precursor type in theadjacent syringes.

The apparatus in FIG. 10 further includes several capillaries 840, shownin dashed outlines. The capillaries are movable, individually or as agroup, in x-, y-, and z-direction. The capillaries have a diameter andan orifice suitable for pressing gas through the capillary to let itstream onto a surface positioned at a distance from the opening. In theexample of FIG. 10, the gas jet hits the passive surface of thesemiconductor chip flip-assembled on the substrate of assembly 930.Preferably, the gas is cooled to a temperature below ambienttemperature. From the cooled surface location, the cooling effectspreads through the semiconductor thickness of the chip and the solderbodies to the substrate.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription.

As an example, the embodiments are effective in semiconductor chips andany other chips with contact pads, which have to undergo assembly on asubstrate or printed circuit board followed, including the process ofunderfilling the gap between chip and substrate. As another example, thesemiconductor devices may include products based on silicon, silicongermanium, gallium arsenide and other semiconductor materials employedin manufacturing. As yet another example, the concept of the inventionis effective for many semiconductor device technology nodes and notrestricted to a particular one.

As another example, the method disclosed can be applied to void-freefilling of gaps between any substrates or other external parts which areinterconnected by spacing elements or otherwise kept at a distance, whendifferent viscosities of adjacent filling materials can be formed tocontrol capillary action. The method can generally be applied tofabricate void-free fillings between solid parts (including glasspates), when portions of the parts can be kept at temperatures differentfrom the remainder of the parts, or when syringes with a precursor atdifferent temperatures are used, or when syringes with precursors ofdifferent viscosities are employed.

It is therefore intended that the appended claims encompass any suchmodifications or embodiments.

1. A method for assembling a semiconductor device comprising the stepsof: providing a semiconductor chip and a substrate assembly, havingsolder joints bonding the chip and the substrate and a gapthere-between; providing an adhesive polymer precursor having aviscosity variable with temperature; providing a heatable plate withmesas; providing a movable gas-blowing capillary assembly; providing amovable precursor-dispensing syringe assembly; placing thechip-and-substrate assembly on a heated mesa to raise the assemblytemperature; placing the capillary assembly near edge locations of thechip-and-substrate assembly; blowing gas of a temperature from thecapillary to lower the local temperature at the edge locations, creatinga temperature profile across the chip-substrate assembly; positioningthe syringe assembly near a first chip edge; depositing a quantity ofthe precursor at the first chip edge to cause the precursor to flow intothe gap; and maintaining a substantially linear front of the precursorflow towards a chip edge opposite the first chip edge by maintaining atemperature profile that is hotter away from the capillary assembly andcooler near the capillary assembly.
 2. The method according to claim 1wherein the substantially linear front of the precursor progressesthrough the gap until the gap is filled with polymer, substantiallywithout voids.
 3. The method according to claim 1 wherein thesemiconductor chip is a multi-chip arrangement.
 4. The method accordingto claim 1 wherein the syringe includes multiple syringes, each syringesuitable for dispensing the precursor at a different temperature.
 5. Themethod according to claim 1 wherein the syringe includes multiplesyringes, each syringe suitable for dispensing a precursor of differentviscosity.
 6. The method according to claim 1 wherein the precursor is apolymer including inorganic filler particles.
 7. The method according toclaim 6 wherein the largest diameter of the filler particles is lessthan approximately 30% of the gap.
 8. The method according to claim 1wherein the blowing gas is cooled to a controlled temperature.
 9. Amethod for assembling a semiconductor device, comprising the steps of:providing a semiconductor chip and a substrate assembly, having a gapbetween the chip and the substrate; creating a non-uniform temperatureprofile across the chip-substrate assembly; and dispensing a quantity ofan adhesive precursor at a first chip edge and thereby creating aprecursor front advancing in the gap to a second ship edge opposite thefirst chip edge.
 10. The method according to claim 9 wherein the step ofcreating the temperature profile includes the steps of: placing thechip-substrate assembly on a heated pedestal; and blowing cooled gasonto selected locations across the assembly.
 11. A method for assemblinga semiconductor device, comprising the steps of: providing asemiconductor chip and a substrate assembly, having a gap between thechip and the substrate; providing an adhesive polymer precursor having aviscosity variable with temperature; dispensing, at a first chip edge, afirst quantity of the precursor at a first temperature, thereby forminga front advancing in the gap to a second chip edge opposite the firstchip edge; and dispensing, at the first chip edge, a second quantity ofthe precursor at a second temperature, thereby forming a front advancingin the gap to the second chip edge opposite the first chip edge.
 12. Themethod according to claim 11 wherein the fronts of the first and thesecond temperature precursors create a substantially linear front ofadvancing precursor.
 13. A method for assembling a semiconductor device,comprising the steps of: providing a semiconductor chip and a substrateassembly, having a gap between the chip and the substrate; providing afirst adhesive polymer precursor having a first viscosities; providing asecond adhesive polymer precursor having a second viscosity; dispensing,at a first chip edge, a quantity of the first precursor, thereby forminga front advancing in the gap to a second chip edge opposite the firstchip edge; and dispensing, at the first chip edge, a quantity of thesecond precursor, thereby forming a front advancing in the gap to asecond chip edge opposite the first chip edge.
 14. The method accordingto claim 13 wherein the fronts of the first and the second precursorscreate a substantially linear front of advancing precursor.
 15. Anapparatus for semiconductor flip-chip assembly comprising: a platehaving a body and a surface heatable to a controlled temperature; mesasprotruding from the surface, each mesa configured to support at leastportion of a semiconductor assembly, the mesas having the sametemperature as the plate; capillaries above the mesas, the capillariesmovable in three dimensions, each capillary having an opening suitablefor flowing gas; and at least one syringe movable in three dimensions,the syringe having an opening suitable for dispensing a polymerprecursor.
 16. The apparatus according to claim 15 wherein the gas isflowing at a controlled rate.
 17. The apparatus according to claim 15wherein the gas is controllably cooled to a temperature from about −10to +20° C.
 18. The apparatus according to claim 15 wherein the polymerprecursor is at a controlled temperature.